There have heretofore been used optical thin films of various compositions in a wide range of application fields. In particular, optical thin films utilizing a frequency-selective transmission or reflection based on optical absorption or interference have been known for a long time. Late years, in the fields of optoelectronics and optronics utilizing laser beams, various developments for creating innovative high-performance optical thin films are driven forward to meet the needs of achieving multi-parallel high-speed information processing utilizing optical multiplicity and of obtaining enhanced phenomenon in optical nonlinear effect or opto-electric effect.
An organic optical material is regarded as one of noteworthy materials or compositions thereof for forming such a new high-performance optical thin film. Generally, the organic material has an advantageous feature of allowing a thin film to be readily produced on a large scale through a synthetic process at a reduced cost. In addition, the organic material advantageously provides an extended range of choice of thin film materials.
Lately, the organic material also comes to the front as the material of high-density recording media.
Specifically, an optical recording medium typified by DVD, CD-R or CD-RW has been recently developed with high priority as high-density information-recording media. In particular, a rewritable optical recording material can be used for various purposes because of its repeatability in information writing and erasing, and developments therefor are actively being made.
The mainstream material of available recording media is an inorganic material typified by chalcogenide materials. In a recording medium using these materials, information is recorded thereon in a heat mode. Thus, a desirably increased recording density cannot be practically obtained therein due to inevitable thermal diffusion in the material.
A specific organic compound, so-called photochromic material, has a property in which its molecular structure is changed through a photoisomerization reaction caused by absorbing a specific light while creating an additional optical absorption capacity (mainly in a visible region), and the photoisomerized material is returned to its original structure through a reverse reaction caused by absorbing a different light. Thus, the photochromic material is expected as a new material of optical recording media for recording, reading and erasing information in a photochemical process (photon mode) including no thermal effect. In the photochromic recording media, information is recorded in a molecular level through photoreaction, molecule, and the photochromic molecule itself has a potential for providing high resolution. Further, a stable molecular material such as diarylethene-based materials can be used to allow even an extremely small record mark to be recorded without instability. Therefore, the photochromic material would be considered as an optimal material of super-high-density recording media using a near-field light (evanescent light).
The inventors proposed an optical recording medium production method using an organic compound, wherein a volatile substance is dispersed in a specific polymer having an affinity to the volatile substance molecules at a high concentration while maintaining a specific function of the volatile substance molecules (see the following Nonpatent Literature-1 and Patent Publications 1 to 3).
[Nonpatent Literature-1]
T. Hiraga, G Chen, K. Tsujita, N. Tanaka, Q. Chen and T. Moriya, A Novel Formation Method of Thin Polymer Film with Densely Dispersed Organic Dye by using Vacuum Technique, “Molecular Crystals and Liquid Crystals”, Gordon and Breach Science Publishers (New York), published 2000, Vol. 344, page 211.
[Patent Publication-1]
Japanese Patent Laid-Open Publication No. 2000-256877
[Patent Publication-2]
Japanese Patent Laid-Open Publication No. 2000-281821
[Patent Publication-3]
Japanese Patent Laid-Open Publication No. 2001-026884
In application of a functional organic compound such as photochromic compounds to an optical recording medium, it is required to disperse or infiltrate organic compound molecules in a macromolecule or silica gel-like matrix at a high concentration. This process has been performed using a spin coating method in many cases.
While there are various wet-type thin film forming processes using solution, dispersion liquid or developer, for example, a coating method such as spin coating, roll coating, dipping or spraying; a printing method such as planographic printing, relief printing, intaglio printing, stencil printing, screen printing or decalcomania; an electrochemical technique such as electrodeposition process, electrolytic polymerization process or micelle electrolytic process; and a Langmuir projecting process of transferring a monomolecular film formed on the water surface, it is extremely difficult to form a polymer film having functional organic compound molecules while maintaining its homogeneity, using these processes. Further, if it is attempted to disperse organic compound molecules at a high concentration, the organic compound molecules will agglutinate, resulting in disappearance of a specific property of the organic compound molecules.
The conventional optical recording medium also has a problem of deterioration in recording accuracy caused by increased recording density.